Not much, fortunately. Longtime G+ photo community maven Trey Ratcliff has posted a quick overview for people making the transition.
From his G+ post:
- Effectively, nothing really changes because you can still upload photos and share them on Google+ publicly or in communities.
- The new Google Photos is just an improved interface which I like way better because I can upload once and share anywhere.
- People can still comment on photos that are posted to G+ or Communities… hashtags, tagging, and all that stuff still works…
- You can still access all your old photos and videos on Google+ from the photos tab of your profile page.
- None of your old comments and +1’s will go away (where did people get that from?)